1(How to avoid) Botching up ioctls 2================================= 3 4From: http://blog.ffwll.ch/2013/11/botching-up-ioctls.html 5 6By: Daniel Vetter, Copyright © 2013 Intel Corporation 7 8One clear insight kernel graphics hackers gained in the past few years is that 9trying to come up with a unified interface to manage the execution units and 10memory on completely different GPUs is a futile effort. So nowadays every 11driver has its own set of ioctls to allocate memory and submit work to the GPU. 12Which is nice, since there's no more insanity in the form of fake-generic, but 13actually only used once interfaces. But the clear downside is that there's much 14more potential to screw things up. 15 16To avoid repeating all the same mistakes again I've written up some of the 17lessons learned while botching the job for the drm/i915 driver. Most of these 18only cover technicalities and not the big-picture issues like what the command 19submission ioctl exactly should look like. Learning these lessons is probably 20something every GPU driver has to do on its own. 21 22 23Prerequisites 24------------- 25 26First the prerequisites. Without these you have already failed, because you 27will need to add a 32-bit compat layer: 28 29 * Only use fixed sized integers. To avoid conflicts with typedefs in userspace 30 the kernel has special types like __u32, __s64. Use them. 31 32 * Align everything to the natural size and use explicit padding. 32-bit 33 platforms don't necessarily align 64-bit values to 64-bit boundaries, but 34 64-bit platforms do. So we always need padding to the natural size to get 35 this right. 36 37 * Pad the entire struct to a multiple of 64-bits if the structure contains 38 64-bit types - the structure size will otherwise differ on 32-bit versus 39 64-bit. Having a different structure size hurts when passing arrays of 40 structures to the kernel, or if the kernel checks the structure size, which 41 e.g. the drm core does. 42 43 * Pointers are __u64, cast from/to a uintprt_t on the userspace side and 44 from/to a void __user * in the kernel. Try really hard not to delay this 45 conversion or worse, fiddle the raw __u64 through your code since that 46 diminishes the checking tools like sparse can provide. The macro 47 u64_to_user_ptr can be used in the kernel to avoid warnings about integers 48 and pointres of different sizes. 49 50 51Basics 52------ 53 54With the joys of writing a compat layer avoided we can take a look at the basic 55fumbles. Neglecting these will make backward and forward compatibility a real 56pain. And since getting things wrong on the first attempt is guaranteed you 57will have a second iteration or at least an extension for any given interface. 58 59 * Have a clear way for userspace to figure out whether your new ioctl or ioctl 60 extension is supported on a given kernel. If you can't rely on old kernels 61 rejecting the new flags/modes or ioctls (since doing that was botched in the 62 past) then you need a driver feature flag or revision number somewhere. 63 64 * Have a plan for extending ioctls with new flags or new fields at the end of 65 the structure. The drm core checks the passed-in size for each ioctl call 66 and zero-extends any mismatches between kernel and userspace. That helps, 67 but isn't a complete solution since newer userspace on older kernels won't 68 notice that the newly added fields at the end get ignored. So this still 69 needs a new driver feature flags. 70 71 * Check all unused fields and flags and all the padding for whether it's 0, 72 and reject the ioctl if that's not the case. Otherwise your nice plan for 73 future extensions is going right down the gutters since someone will submit 74 an ioctl struct with random stack garbage in the yet unused parts. Which 75 then bakes in the ABI that those fields can never be used for anything else 76 but garbage. This is also the reason why you must explicitly pad all 77 structures, even if you never use them in an array - the padding the compiler 78 might insert could contain garbage. 79 80 * Have simple testcases for all of the above. 81 82 83Fun with Error Paths 84-------------------- 85 86Nowadays we don't have any excuse left any more for drm drivers being neat 87little root exploits. This means we both need full input validation and solid 88error handling paths - GPUs will die eventually in the oddmost corner cases 89anyway: 90 91 * The ioctl must check for array overflows. Also it needs to check for 92 over/underflows and clamping issues of integer values in general. The usual 93 example is sprite positioning values fed directly into the hardware with the 94 hardware just having 12 bits or so. Works nicely until some odd display 95 server doesn't bother with clamping itself and the cursor wraps around the 96 screen. 97 98 * Have simple testcases for every input validation failure case in your ioctl. 99 Check that the error code matches your expectations. And finally make sure 100 that you only test for one single error path in each subtest by submitting 101 otherwise perfectly valid data. Without this an earlier check might reject 102 the ioctl already and shadow the codepath you actually want to test, hiding 103 bugs and regressions. 104 105 * Make all your ioctls restartable. First X really loves signals and second 106 this will allow you to test 90% of all error handling paths by just 107 interrupting your main test suite constantly with signals. Thanks to X's 108 love for signal you'll get an excellent base coverage of all your error 109 paths pretty much for free for graphics drivers. Also, be consistent with 110 how you handle ioctl restarting - e.g. drm has a tiny drmIoctl helper in its 111 userspace library. The i915 driver botched this with the set_tiling ioctl, 112 now we're stuck forever with some arcane semantics in both the kernel and 113 userspace. 114 115 * If you can't make a given codepath restartable make a stuck task at least 116 killable. GPUs just die and your users won't like you more if you hang their 117 entire box (by means of an unkillable X process). If the state recovery is 118 still too tricky have a timeout or hangcheck safety net as a last-ditch 119 effort in case the hardware has gone bananas. 120 121 * Have testcases for the really tricky corner cases in your error recovery code 122 - it's way too easy to create a deadlock between your hangcheck code and 123 waiters. 124 125 126Time, Waiting and Missing it 127---------------------------- 128 129GPUs do most everything asynchronously, so we have a need to time operations and 130wait for outstanding ones. This is really tricky business; at the moment none of 131the ioctls supported by the drm/i915 get this fully right, which means there's 132still tons more lessons to learn here. 133 134 * Use CLOCK_MONOTONIC as your reference time, always. It's what alsa, drm and 135 v4l use by default nowadays. But let userspace know which timestamps are 136 derived from different clock domains like your main system clock (provided 137 by the kernel) or some independent hardware counter somewhere else. Clocks 138 will mismatch if you look close enough, but if performance measuring tools 139 have this information they can at least compensate. If your userspace can 140 get at the raw values of some clocks (e.g. through in-command-stream 141 performance counter sampling instructions) consider exposing those also. 142 143 * Use __s64 seconds plus __u64 nanoseconds to specify time. It's not the most 144 convenient time specification, but it's mostly the standard. 145 146 * Check that input time values are normalized and reject them if not. Note 147 that the kernel native struct ktime has a signed integer for both seconds 148 and nanoseconds, so beware here. 149 150 * For timeouts, use absolute times. If you're a good fellow and made your 151 ioctl restartable relative timeouts tend to be too coarse and can 152 indefinitely extend your wait time due to rounding on each restart. 153 Especially if your reference clock is something really slow like the display 154 frame counter. With a spec lawyer hat on this isn't a bug since timeouts can 155 always be extended - but users will surely hate you if their neat animations 156 starts to stutter due to this. 157 158 * Consider ditching any synchronous wait ioctls with timeouts and just deliver 159 an asynchronous event on a pollable file descriptor. It fits much better 160 into event driven applications' main loop. 161 162 * Have testcases for corner-cases, especially whether the return values for 163 already-completed events, successful waits and timed-out waits are all sane 164 and suiting to your needs. 165 166 167Leaking Resources, Not 168---------------------- 169 170A full-blown drm driver essentially implements a little OS, but specialized to 171the given GPU platforms. This means a driver needs to expose tons of handles 172for different objects and other resources to userspace. Doing that right 173entails its own little set of pitfalls: 174 175 * Always attach the lifetime of your dynamically created resources to the 176 lifetime of a file descriptor. Consider using a 1:1 mapping if your resource 177 needs to be shared across processes - fd-passing over unix domain sockets 178 also simplifies lifetime management for userspace. 179 180 * Always have O_CLOEXEC support. 181 182 * Ensure that you have sufficient insulation between different clients. By 183 default pick a private per-fd namespace which forces any sharing to be done 184 explicitly. Only go with a more global per-device namespace if the objects 185 are truly device-unique. One counterexample in the drm modeset interfaces is 186 that the per-device modeset objects like connectors share a namespace with 187 framebuffer objects, which mostly are not shared at all. A separate 188 namespace, private by default, for framebuffers would have been more 189 suitable. 190 191 * Think about uniqueness requirements for userspace handles. E.g. for most drm 192 drivers it's a userspace bug to submit the same object twice in the same 193 command submission ioctl. But then if objects are shareable userspace needs 194 to know whether it has seen an imported object from a different process 195 already or not. I haven't tried this myself yet due to lack of a new class 196 of objects, but consider using inode numbers on your shared file descriptors 197 as unique identifiers - it's how real files are told apart, too. 198 Unfortunately this requires a full-blown virtual filesystem in the kernel. 199 200 201Last, but not Least 202------------------- 203 204Not every problem needs a new ioctl: 205 206 * Think hard whether you really want a driver-private interface. Of course 207 it's much quicker to push a driver-private interface than engaging in 208 lengthy discussions for a more generic solution. And occasionally doing a 209 private interface to spearhead a new concept is what's required. But in the 210 end, once the generic interface comes around you'll end up maintainer two 211 interfaces. Indefinitely. 212 213 * Consider other interfaces than ioctls. A sysfs attribute is much better for 214 per-device settings, or for child objects with fairly static lifetimes (like 215 output connectors in drm with all the detection override attributes). Or 216 maybe only your testsuite needs this interface, and then debugfs with its 217 disclaimer of not having a stable ABI would be better. 218 219Finally, the name of the game is to get it right on the first attempt, since if 220your driver proves popular and your hardware platforms long-lived then you'll 221be stuck with a given ioctl essentially forever. You can try to deprecate 222horrible ioctls on newer iterations of your hardware, but generally it takes 223years to accomplish this. And then again years until the last user able to 224complain about regressions disappears, too. 225
